Oh right I forgot you can just search. In that case you don't even need the "otherinstance.com" part
You can just search for "@personiwannafollow" and they'll show up and you can hit a single button to follow.
It's the big, and only, search bar at the top of the home page...
I think it means going to your home instance, searching for @[email protected] and clicking the follow button that will show up. But maybe you mean something else? Either way it's not easy to find it.